The application requests funding for a carefully controlled study of identical twins to identify and clarify interrelationships between risk indicators, disc degeneration, other spine pathology, and back pain. A better understanding of these relationships is necessary for the development of rational, effective preventive and therapeutic approaches to back pain. The use of appropriately selected identical twin pairs offers a rare opportunity to study the effects of specific risk factors while controlling the effects of genetic makeup and many other potentially confounding factors. Such a carefully controlled study design also makes the use of magnetic resonance imaging financially feasible since smaller numbers of subjects can be utilized to test hypotheses. The Finnish Twin Cohort, a population-based twin registry that is unique in the world, will be used. Questionnaire data gathered over the past 15 years from the cohort provides an accessible database on long-term environmental exposures of interest, as well as other potentially important covariates. A series of six independent investigations will be conducted. Each will determine the effects of a specific long-term environmental exposure on disc degeneration and other spinal pathology, as assessed by magnetic resonance imaging. The environmental exposures to be examined will be: (1) cigarette smoking; (2) regular aerobic exercise; (3) vehicular vibration without materials handling; (4) vehicular vibration with materials handling; (5) prolonged sitting; and (6) heavy occupational loading. Approximately 30 twin pairs (60 subjects), discordant for the factor of interest, will serve as subjects for each investigation. Extensive interview data on possible risk factors and covariates, as well as functional measurements, will be obtained for all subjects at the time of the magnetic resonance imaging. Subsequent back pain experiences will be assessed after one year through a follow-up questionnaire. The overall goal is to identify risk factors for back problems, and the mechanics through which they affect spine pathology and symptoms. A better understanding of the relative effects of individual and environmental factors on the degenerative process in the spine also is anticipated. The study should provide clear insights into the basic, yet elusive, issues of back pain etiology and risk factors.